1. Field of the Invention
This invention concerns a process for the continuous separation of slightly volatile components from a thermally pre-treated crude phthalic acid anhydride, produced by naphthalene oxidation, before its pure distillation.
2. Description of the Prior Art
Crude phthalic acid anhydride contains varying amounts of non-volatile impurities of different kinds depending on the initial substance from which it was produced and on the oxidation process which was used for its production. These impurities interfere, for different reasons, in the purification of the crude product by rectification. Therefore, in conventional processes, the crude product is subjected to a thermal pre-treatment before the rectification, and as a result of such thermal pretreatment, dibasic acids are transformed into their anhydrides, mono-carboxylc acids are decarboxylated and naphthoquinone is transformed into higher-molecular weight products which are non-volatile or only slightly volatile impurities. While the portion of the high-molecular weight impurities is low in the case of a crude oxidation product of o-xylol, and practically no problems arise in the rectification of such a product; crude phthalic acid anhydride produced from naphthalene contains, among other ingredients, generally 1.0 to 1.3 percent by weight of non-volatile impurities and up to 0.5 percent by weight naphthoquinone. Since the naphthoquinone is transformed into higher-molecular weight products, which are impurities of low volatility, the phthalic acid anhydride finally supplied to the rectification may contain up to about 2 percent by weight of non-volatile impurities. As used herein, the terms non-volatile impurities and impurities refer to impurities that are non-volatile or only slightly volatile under the conditions of temperature and pressure involved in the described process.
It has been determined that these impurities can interfere in the continuous rectification of crude phthalic acid anhydride, which is generally performed in two stages. Such interference is caused by the settling of the impurities on the heating surfaces of the evaporators connected to the rectification column. As a result of this interference, the operation is generally periodically interrupted in order to clean the evaporators. To resolve this problem, it has been proposed to initially subject the crude phthalic acid anhydride to distillation in batches, in order to separate the impurities from the phthalic acid anhydride. (DE-AS No. 1,668,398). Alternatively, a three-stage distillation of phthalic acid anhydride from naphthalene has been described in connection with a continuous operating method where, in the first stage, the impurities are separated initially by evaporating the crude product (H. Suter, Phthalic Acid Anhydride and Its Use, 1972, P. 45). In this three-stage method, a concentration of the impurities occurs in the first evaporation stage which leads quickly to a deposit on the heating surfaces. Finally, it is known to initially distill the pre-treated, liquid crude phthalic acid anhydride under reduced pressure, then to process the distillation residue in a separate residue distillation and to combine the distillates of the two procedures. With this operating method, the crude product must be heated to a high temperature, i.e., 450.degree. C., in order to achieve a quantitative evaporation of the phthalic acid anhydride from the crude product. The high temperature favors the decomposition of phthalic acid anhydride and causes a reduction in the yield of the pure product. Furthermore, a correspondingly high pressure is required with the heating to 450.degree. C. in order to keep the crude product in a liquid state in the heat exchanger, resulting in a need for additional equipment. If one would not apply the increased pressure, the phthalic acid anhydride would start boiling in the exchanger whereby impurities would again separate on the heat exchanger surfaces (German Disclosure Publication No. 1,295,540 corresponding to U.S. Pat. No. 3,380,896).